1. Field of the Invention
The present invention relates to a semiconductor laser device used for optical communications and, more particularly, to a semiconductor laser device having an asymmetrical optical intensity distribution.
2. Description of the Related Art
With the spread of public networks using optical fibers, there is an increasing need to transmit a large amount of information at low cost. To meet such a need, the public networks must be improved at low cost to increase the amount of information that can be transmitted. This requires the development of a semiconductor laser device which has good optical matching to the existing optical fiber networks and high optical output efficiency.
FIG. 15 is a cross-sectional view of a conventional semiconductor laser.
In the figure, reference numeral 200 denotes a semiconductor laser; 202 a p conductivity type InP substrate (“p conductivity type” and “n conductivity type” are hereinafter expressed as “p-” and “n-”, respectively); 204 a p-InP cladding layer; and 206 an active region. The active region 206 comprises: an active layer having a multiple quantum well structure made of InGaAsP well layers and InGaAsP barrier layers; and light confining InGaAsP layers disposed over and under the active layer, sandwiching the active layer. Reference numeral 208 denotes an n-InP cladding layer; 210 an InP current blocking layer; 212 an n-InP contact layer, 214 an n type electrode; and 216 a p type electrode.
In the semiconductor laser 200, if a bias voltage is applied between the p type electrode 216 and then type electrode 214 such that the p type electrode 216 is at a positive potential, a current effectively flows through only the active region 206, making it possible for the semiconductor laser 200 to operate with low current and emit light. Since the refractive index of the InP cladding layers 204 and 208 are smaller than that of the active region 206, the light is distributed mainly in and around the active region 206.
FIG. 16 is a schematic diagram showing a near-field pattern of a conventional semiconductor laser. In the figure, the vertical axis indicates the intensity of the light, while the horizontal axis indicates the distance from the active region 206. The broken line parallel to the horizontal axis indicates 10% of the peak value of the optical intensity.
In FIG. 16, the optical intensity distribution is symmetrical about the active region 206 at the center (the optical intensity distribution on the n side is identical to that on the p side). In the example shown in FIG. 16, the areas defined by the optical intensity distribution curves on the n side and on the p side are each 0.397. The optical intensity distribution curve approaches the horizontal axis as the distance from the active region 206 increases. The optical intensity becomes substantially zero at positions approximately 2.0 μm away from the active region 206 at the center.
Two prior art techniques for semiconductor lasers are described as follows.
One was devised for high-power double heterostructure semiconductor lasers in which an active layer having a multiple quantum well structure is sandwiched by an n-AlGaAs optical guide layer and a p-AlGaAs optical guide layer which, in turn, are sandwiched by an n-AlGaAs cladding layer and a p-AlGaAs cladding layer. These layers have different material composition ratios. With this arrangement, the total refraction distribution is shifted toward one or the other side of the active layer such that the peak of the optical density distribution does not coincide with that of the electric current, thereby preventing degradation of the materials and enhancing the reliability (for example, see Japanese Patent Laid-Open No. Hei 11 (1999)-243259, pp. 6-9, FIGS. 1-3).
The other technique was developed for semiconductor lasers used as an excitation light source for optical fiber amplifiers, etc. In semiconductor lasers using this technique, an active layer is sandwiched by upper and lower guide layers which, in turn, are sandwiched by upper and lower cladding layers.
Furthermore, semiconductor layers having a refractive index lower than that of the cladding layers are each inserted between the upper guide layer and the upper cladding layer or between the lower guide layer and the lower cladding layer, thereby realizing a semiconductor laser having a narrow vertical radiation angle (20° C. or less) and a stable transverse mode (for example, see Japanese Patent Laid-Open No. Hei 8(1996)-195529, pp. 3-4, FIG. 3).
When the semiconductor laser 200 is emitting light, a large amount of light leaks into the p-InP cladding layer 204 and the n-InP cladding layer 208 since the active region 206 is a thin layer. Generally, to obtain a high ratio of light output to electric current (that is, slope efficiency) of a semiconductor laser, it is necessary to reduce the amount of light absorbed in the areas outside the active region 206, especially in the p-InP cladding layer 204.
A major factor contributing to light absorption in the p type semiconductor area is that there is noticeable inter-valence band absorption in the p type impurity-injected region. This phenomenon occurs with not only InP type materials but also AlGaAs type materials. However, the phenomenon is noticeably observed in the case of the InP type materials.
To control this phenomenon, conventional methods reduce the carrier concentration of the p type semiconductor area, for example, that of the p-InP cladding layer 204. However, the electric resistance of the p type semiconductor area generally tends to be higher than that of the n type semiconductor area, and reducing the carrier concentration of the p type semiconductor area further increases its electric resistance, entailing the problem of a reduction in the optical output due to heat produced when a large current flows, that is, the problem of roll-off.
Furthermore, since optical fiber networks have already begun to spread, semiconductor lasers must have not only high slope efficiency but also sufficient optical matching to the existing optical fiber networks. Therefore, the semiconductor lasers must have a near-field pattern which is not much different than that of conventional semiconductor lasers.